1. Field of the Invention
The present invention relates to a cycle-based communication system for transmitting useful data between users of the system. The communication system includes a data bus and the users connected to it. The data transmission is effected within cyclically repeating timeframes, each having at least two timeslots. Each timeslot is intended for transmitting one message. One message includes at least some of the useful data, and each message is assigned an identifier.
The invention furthermore pertains to a method for transmitting useful data in a cycle-based communication system between users of the system, over a data bus to which the users are connected. The useful data are transmitted within cyclically repeating timeframes, each having at least two timeslots. In each timeslot, one message is transmitted. At least some of the useful data is stored in a message. Each message is assigned an identifier.
2. Prior Art
From the prior art, it is known to use communication systems of the type defined above in motor vehicles or other means of transportation (such as aircraft, trains, and ships) for exchanging data between control units. The control units serve to control or regulate certain functions of the means of transportation, such as drive functions (for instance, the driving engine and transmission), safety functions (such as anti-lock brake system or ABS, traction control or ASR, and electronic stability program or ESP) or comfort functions (such as air-conditioning of the interior).
In the known communication systems, the data exchange between the control units essentially takes place in serial form over the so-called bus systems. For the data traffic to be reliable and regulated, an agreement about the way in which data will be transmitted, or so-called protocol, is necessary. FlexRay is one such protocol, which makes it possible to transmit serial data with time control over a bus system. The data are packed into a message in a data frame which additionally includes data for controlling and securing the data traffic. These messages are transmitted cyclically in a fixedly predetermined order, or so-called timeslots.
In FlexRay, one timeframe (basic cycle) comprises timeslots that are fixed in each fundamental cycle (for high-priority messages), and variable timeslots (for low-priority messages or for messages with an altered or longer cycle time). The division of a fundamental cycle into fixed and variable timeslots is freely selectable and is influenced by the requirements of the data transmission. The requirements are predetermined for instance by the application within which the data transmission is to be effected.
In FlexRay, according to the prior art, different messages can be transmitted within one fundamental cycle only at different times. Depending on the number of messages to be transmitted, the length of one fundamental cycle is thus determined. Even if a plurality of low-priority messages need to be transmitted only every nth fundamental cycle, still a waiting time must be reserved at least for each of these messages. Thus, the timeframe includes as many timeslots as there are different messages to be transmitted in any one fundamental cycle. If a message is not transmitted in a certain cycle, the timeslot for this message remains empty in that cycle.
The shortest repetition time for “fast” messages, that is, messages to be transmitted often, accordingly depends on the fundamental cycle. The longer the fundamental cycle, the more infrequently can “fast” messages be sent. To allow the “fast” messages to be repeated more often despite a relatively long fundamental cycle, it is known to assign the messages a plurality of timeslots within one fundamental cycle. However, this has the disadvantage that a strict periodicity can be attained only with difficulty, if at all, and that in the implementation, the memory requirement may increase, since a plurality of message objects must be procured for one message.
From Lönn, H. et al: Synchronisation in Safety-Critical Distributed Control Systems, Algorithms and Architectures for Parallel Processing, 1995, ICAPP 95, IEEE First ICAPP, IEEE First International Conference on Brisbane, Queensland, Australia, Apr. 19 to 21, 1995, New York, N.Y., USA, IEEE, US, Apr. 19, 1995, pages 891-899, ISBN 0-7803-2018-2, a cycle-based communication system of the type defined above for transmitting useful data between users of the system is known. However, no indication can be found in this publication that in a timeslot of the timeframe in which messages are transmitted with the useful, messages from different users of the system can be transmitted in different transmission cycles on the order of multiplexing.
Belschner, R. et al: Anforderungen an ein zukünftiges Bussystem für fehlertolerante Anwendungen aus Sicht Kfz-Hersteller [Requirements of a Future Bus System for Error-Tolerant Applications from the Standpoint of Motor Vehicle Manufacturers], VDI-Berichte, Verein Deutscher Ingenieure—Gesellschaft Fahrzeug-und Verkehrstechnik [VDI Reports, Association of German Engineers—Society for Vehicular and Traffic Technology], Vol. 1547, Oct. 6, 2000, pages 23-41, generally describes the requirements of motor vehicle manufacturers for a future communication system. However, no indication of multiplexing the messages in the timeslots of the timeframes can be found in this publication.
Johansson, L. et al: QRcontrol, a Bit-Oriented Communication Concept for Control Systems, QRtech Publication, Jan. 2, 2001, describes a cycle-based communication system for transmitting useful data between users of the system. The communication system described differs from that described above, in particular in that the known system uses 1-bit-long timeslots. If a user wishes to send a plurality of bits, it requires a plurality of timeslots for the purpose. Moreover, the known communication system makes completely different timeslot sequences per transmission cycle possible. Thus, the number of booked timeslots per user consequently changes as a function of the current cycle. In this reference, there is accordingly no fixed chronological association between a timeslot and a user. This lack of a fixed chronological association of the timeslots with specific users of the system represents an important distinction from the communication system of the type defined at the outset above.
The object of the present invention, data transmission via a cyclical time-based communication system, in which messages are transmitted in timeslots of a fixed length in cyclically repeating timeframes, is to assure optimal support by the protocol for various period lengths.
For attaining this object, the present invention, based on the communication system of the type defined at the outset, proposes that the identifier is stored in the message as part of the message; that the or every message includes data about the cycle; that the timeslots have a fixed length; and that at least one of the timeslots of the timeframes can be used in various cycles for transmitting different messages.